This invention relates to a novel hydrogenation catalyst which is stable in air at room temperature and has a high activity even in the absence of a reducing metal compound, to a process for hydrogenating a conjugated diene polymer with this hydrogenation catalyst and to a hydrogenated polymer obtained by the hydrogenation process. More particularly, this invention relates to a titanocene diaryl compound as a hydrogenation catalyst, to a process for preferentially hydrogenating the unsaturated double bonds of the conjugated diene units of a conjugated diene polymer with this compound under mild hydrogenation conditions and to a hydrogenated, conjugated diene-styrene block copolymer produced by this process which is a useful thermoplastic elastomer.
For the hydrogenation of compounds having olefinically unsaturated double bonds various catalysts are known. There are generally two types of hydrogenation catalysts, namely, (1) heterogeneous system catalysts of the carrier-supported type wherein a metal such as Ni, Pt, Pd, Ru or the like is supported on a carrier such as carbon, silica, alumina, diatomaceous earth or the like and (2) homogeneous system catalysts such as (a) the so-called Ziegler catalysts using a combination of an organic acid salt or acetylacetone salt of Ni, Co, Fe, Cr or the like and a reducing agent such as an organoaluminum or the like and (b) the so-called organic complex catalysts such as an organometallic compound of Ru, Rh or the like. The heterogeneous system catalysts are in wide use in industry, but as compared with the homogeneous system catalysts, they are generally low in activity; therefore, in carrying out a desired hydrogenation using such a heterogeneous system catalyst, a large amount of the catalyst is required, and the hydrogenation must be effected at a high temperature at a high pressure. Therefore, the use of the heterogenous system catalyst is not economical. On the other hand, in the case of the homogeneous system catalysts, the hydrogenation proceeds generally in a homogeneous system; therefore, as compared with the heterogeneous system catalysts, the homogeneous system catalysts have a high activity, and a small amount of the catalyst is sufficient and the hydrogenation can be carried out at a lower temperature at a lower pressure. However, the homogeneous system catalysts have the drawbacks that the process for preparing them is complicated, the stability of the catalysts cannot be said to be sufficient, the reproduction is inferior and unfavorable side reactions tend to be caused. Hence, the development of a hydrogenation catalyst which has a high activity and is easy to handle has been strongly desired.
Polymers obtained by subjecting a conjugated diene to polymerization or copolymerization are widely used as an elastomer in industry. These polymers have the remaining unsaturated double bonds in the polymer chain, and the double bonds are advantageously utilized in vulcanization on one hand, but render the polymers poor in stability such as weather resistance and oxidation resistance. As a typical example, block copolymers obtained from a conjugated diene and a vinyl-substituted aromatic hydrocarbon are used in the unvulcanized state as a thermoplastic elastomer, a transparent impact-resistant resin, or a modifier for styrene resins and olefin resins; however, these copolymers are inferior in weather resistance, oxidation resistance and ozone resistance because of the unsaturated double bonds remaining in the polymer chain, which makes the copolymers unsuitable for use in the field of outer-coating materials wherein the performances are required, and the use thereof is limited.
The above inferior stability in weather resistance, oxidation resistance and ozone resistance can be remarkably improved by hydrogenating the polymers to saturate the unsaturated double bonds remaining in the polymer chain. Many processes have been proposed for hydrogenating polymers having unsaturated double bonds. In general, there are known two processes, namely, a process using the abovementioned carrier-supported type heterogeneous system catalyst (1) and a process using the above-mentioned Ziegler type homogeneous system catalyst (2). These catalysts have the above-mentioned features, respectively, and, when used in polymer-hydrogenation, they have the following further features as compared with the hydrogenation of low molecular weight compounds.
In the process using a heterogeneous system catalyst of the carrier-supported type, the hydrogenation reaction is caused by the contact of the catalyst with a polymer; therefore it becomes difficult for the reactants to contact with the catalyst owing to the viscosity of the reaction system, the steric hindrance of polymer chain, etc. Hence, for efficient hydrogenation of a polymer, a larger amount of a catalyst and more severe conditions are required, so that the decomposition and gelation of polymer tend to take place and simultaneously, the energy cost becomes higher. Further, in the hydrogenation of a copolymer of a conjugated diene and a vinyl-substituted aromatic hydrocarbon, even the aromatic nucleus portions of the copolymer are usually hydrogenated and it is difficult to selectively hydrogenate only the unsaturated double bonds of the conjugated diene units. Furthermore, the physical removal of the catalyst after hydrogenation from the hydrogenated polymer is extremely difficult and complete removal is substantially impossible.
On the other hand, in the process using a homogeneous system catalyst of the Ziegler type, the hydrogenation reaction usually proceeds in a homogeneous system; therefore, the catalyst activity is high and the hydrogenation can be effected under mild conditions. In addition, by selecting appropriate hydrogenation conditions, it is possible to preferentially hydrogenate, to a considerable extent, the unsaturated double bonds of the conjugated diene units of a copolymer of a conjugated diene and a vinyl-substituted aromatic hydrocarbon. However, in the hydrogenation of a copolymer of a conjugated diene and a vinyl-substituted aromatic hydrocarbon, preferential hydrogenation of the unsaturated double bonds of the conjugated diene units cannot sufficiently be done. Moreover, homogeneous system hydrogenation catalysts of the Ziegler type which are currently available are expensive and, in the removal of the remaining catalyst from a hydrogenation product (this removal is necessary because the remaining catalyst adversely affects the stability of the hydrogenated product), a complicated deashing step is generally required. Therefore, for economical hydrogenation, there is strongly required development of a highly active hydrogenation catalyst which is effective even in such a small amount as not to affect adversely the stability of a hydrogenated polymer and require no deashing step, or of a hydrogenation catalyst which can easily be removed from a hydrogenated polymer.
Polymers produced by hydrogenating a block copolymer of a conjugated diene and a vinyl-substituted aromatic hydrocarbon are thermoplastic elastomers having excellent weather resistance and heat resistance and now often used in industry as industrial parts, electrical parts, resin modifiers, etc. However, the polymers are not yet sufficient in elongation, adhesiveness, low-temperature characteristics, balance between mechanical strength and rubber elasticity when blended, stability, processability, etc. Under such circumstances, hydrogenated block copolymers wherein these characteristics have been improved have strongly been desired.